Image forming apparatus capable of changing rotation speed of fixing member

ABSTRACT

An image forming apparatus includes a fixing member configured to fix an image to a medium by heating the medium, a heating member configured to heat the fixing member, a pressure member pressed against the fixing member so as to presses the medium against the fixing member, a first temperature detection unit for detecting a temperature of the fixing member, a second temperature detection unit for detecting a temperature of the pressure member, and a control unit that controls a rotation speed of the fixing member. The control unit controls the rotation speed of the fixing member based on a temperature difference between the temperature detected by the first temperature detection unit and the temperature detected by the second temperature detection unit.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus usingelectrophotography such as a facsimile, a printer, a copier and thelike.

A general image forming apparatus using electrophotography includes afixing unit that fixes a toner image to a sheet by application of heatand pressure. The fixing unit includes a fixing roller having aninternal heat source and a pressure roller pressed against the fixingroller. The sheet to which a toner image is transferred is fed through anip portion between the fixing roller and the pressure roller. When aprint command is received, the image forming apparatus starts rotatingthe fixing roller at the same speed as a printing speed, controls atemperature of the fixing unit, and feeds the sheet through the fixingunit so as to fix the toner image to the sheet.

The fixing unit generally includes temperature sensors for detectingtemperatures of the fixing roller and the pressure roller. When thesheet starts to be fed toward the fixing unit, the heat source startsheating the fixing roller. As the fixing roller is heated, a heatstorage amount gradually increases. Generally, the heat storage amountreaches a sufficient amount for fixing the toner image when the sheetreaches the fixing roller.

In this regard, when the thickness of the sheet is thin, the temperatureof the fixing roller overshoots and finally reaches the targettemperature. Therefore, it is necessary to provide a waiting time beforestarting the feeding of the sheet. In this regard, Japanese Laid-OpenPatent Publication No. H10-104990 discloses a configuration capable ofreducing the waiting time.

However, in the general image forming apparatus, it is difficult toobtain excellent fixing property.

SUMMARY OF THE INVENTION

An aspect of the present invention is intended to provide an imageforming apparatus capable of enhancing fixing property.

According to an aspect of the present invention, there is provided animage forming apparatus including a fixing member configured to fix animage to a medium by heating the medium, a heating member configured toheat the fixing member, a pressure member pressed against the fixingmember so as to presses the medium against the fixing member, a firsttemperature detection unit for detecting a temperature of the fixingmember, a second temperature detection unit for detecting a temperatureof the pressure member, and a control unit that controls a rotationspeed of the fixing member. The control unit controls the rotation speedof the fixing member based on a temperature difference between thetemperature detected by the first temperature detection unit and thetemperature detected by the second temperature detection unit.

With such a configuration, excellent fixing property can be obtained.

According to another aspect of the present invention, there is providedan image forming apparatus including a fixing member heated by a heatingmember, the fixing member being configured to fix an image to a mediumby heating the medium, a pressure member pressed against the fixingmember so as to presses the medium against the fixing member, and acontrol unit that controls a rotation speed of the fixing member. Thecontrol unit causes the fixing member to rotate at a higher speed, as aheat storage amount in the fixing member becomes smaller.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificembodiments, while indicating preferred embodiments of the invention,are given by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a schematic sectional view showing an image forming apparatusaccording to the first embodiment of the present invention;

FIG. 2 is a block diagram showing a control system of the image formingapparatus according to the first embodiment;

FIG. 3 is a schematic view showing a fixing unit according to the firstembodiment;

FIG. 4A is a longitudinal sectional view showing the fixing unitaccording to the first embodiment;

FIGS. 4B and 4C are cross sectional views respectively taken along aline 4B-4B and a line 4C-4C in FIG. 4A;

FIG. 5 is a flowchart showing an operation for controlling a rotationspeed of a fixing unit motor according to the first embodiment;

FIG. 6 is a schematic view for illustrating a relationship between anupper/lower temperature difference ΔT0 and a surface temperaturechanging amount D from start of rotation according to the firstembodiment;

FIG. 7 is a schematic view for illustrating a relationship among theupper/lower temperature difference ΔT0, the surface temperature changingamount D from start of rotation, a heat input amount P, a heat storageamount Q at start of medium passing, and a speed-change-decisioncriterion temperature difference ΔTth according to the first embodiment;

FIG. 8 is a schematic view for illustrating a calculation method of anoptimum pre-arrival rotation speed V_(A) according to the firstembodiment;

FIGS. 9A through 9F are timing charts showing an operation of a fixingunit of a comparison example when an upper/lower temperature differenceΔT0 is large;

FIGS. 9G through 9L are timing charts showing an operation of the fixingunit of the comparison example when the upper/lower temperaturedifference ΔT0 is small;

FIGS. 10A through 10F are timing charts showing an operation of thefixing unit according to the first embodiment;

FIG. 11 is a block diagram showing a control system of an image formingapparatus according to the second embodiment of the present invention;

FIG. 12 is a flowchart showing an operation for controlling a rotationspeed of a fixing unit motor according to the second embodiment;

FIG. 13 is a schematic view for illustrating a relationship between aheat storage amount Q at start of medium passing and a surfacetemperature changing amount D from start of rotation for differentenvironmental temperatures according to the second embodiment;

FIG. 14 is a schematic view for illustrating a relationship among anupper/lower temperature difference ΔT0, the surface temperature changingamount D from start of rotation, a heat input amount P, the heat storageamount Q at start of medium passing, and a speed-change-decisioncriterion temperature difference ΔTth according to the secondembodiment;

FIG. 15 is a schematic view for illustrating a method for calculating anoptimum pre-arrival rotation speed V_(A1), V_(A2) or V_(A3) fordifferent environmental temperatures according to the second embodiment;

FIGS. 16A through 16F are timing charts showing an operation of thefixing unit according to the second embodiment under low temperature andlow humidity environment;

FIGS. 16G through 16L are timing charts showing an operation of thefixing unit according to the second embodiment under high temperatureand high humidity environment;

FIG. 17 is a block diagram showing a control system of an image formingapparatus according to Modification 1 of the second embodiment;

FIG. 18 is a schematic view for illustrating a relationship among anupper/lower temperature difference ΔT0, a surface temperature changingamount D from start of rotation, a heat input amount P, a heat storageamount Q at start of medium passing, and a speed-change-decisioncriterion temperature difference ΔTth according to Modification 1 of thesecond embodiment;

FIG. 19 is a block diagram showing a control system of an image formingapparatus according to Modification 2 of the second embodiment, and

FIG. 20 is a schematic view for illustrating a relationship among anupper/lower temperature difference ΔT0, a surface temperature changingamount D from start of rotation, a heat input amount P, a heat storageamount Q at a start of medium passing, and a speed-change-decisioncriterion temperature difference ΔTth according to Modification 2 of thesecond embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, embodiments of the present invention will be described withreference to drawings. The drawings are provided for illustrativepurpose and are not intended to limit the scope of the presentinvention.

First Embodiment

FIG. 1 is a schematic sectional view showing an image forming apparatus1 according to the first embodiment of the present invention. The imageforming apparatus 1 includes a medium feeding unit 41, an LED head 3(i.e., an exposure unit), a toner image forming unit 5 (i.e., adeveloper image forming unit), a fixing unit 6, and a medium ejectionunit 42. The medium feeding unit 41, the writing sensor 8, the tonerimage forming unit 5, the fixing unit 6, and the medium ejection unit 42are arranged in this order along a medium feeding path 2.

The medium feeding unit 41 is configured to feed a medium M such as apaper to a medium feeding path 2. The LED head 3 is provided adjacent tothe toner image forming unit 5, and configured to emit light so as toexpose a surface of a photosensitive drum 51 (described later) of thetoner image forming unit 5 to form a latent image.

The toner image forming unit 5 includes the photosensitive drum 51(i.e., a image bearing body) that rotates in a predetermined direction(clockwise in FIG. 1), a charging member 52 that uniformly charges thesurface of the photosensitive drum 51, and a developing unit 53 thatdevelops the latent image (formed by the LED head 3) on the surface ofthe photosensitive drum 51 using a toner as a developer. A transfermember 54 is provided so as to face the photosensitive drum 51 via themedium feeding path 2 for transferring a toner image from thephotosensitive drum 51 to the medium M. A writing sensor 8 is providedupstream of the toner image forming unit 5 along the medium feeding path2 for detecting a position of the medium M.

The fixing unit 6 is configured to fix the toner image (having beentransferred to the medium M) to the medium M. The medium ejection unit42 is configured to eject the medium M (to which the toner image isfixed) outside the image forming apparatus 1.

When a printing control unit 100 (FIG. 2) of the image forming unit 1receives a print command, the medium feeding unit 41 feeds the medium Malong the medium feeding path 2 toward the toner image forming unit 5 ata timing in synchronization with image formation by the toner imageforming unit 5. In the toner image forming unit 5, the surface of thephotosensitive drum 51 is uniformly charged by the charging member 52.The LED head 3 emits light according to image data, and a latent imageis formed on the surface of the photosensitive drum 51. The latent imageis developed by the developing unit 53, so that a toner image (i.e., adeveloper image) is formed on the photosensitive drum 51. The tonerimage is transferred from the photosensitive drum 51 to the medium Mwhen the medium M passes a nip portion between the photosensitive drum51 and the transfer member 54. The medium M to which the toner image istransferred is fed to the fixing unit 6. The fixing unit 6 fixes thetoner image to the medium M by application of heat and pressure (i.e., afixing process). The medium ejection unit 42 ejects the medium M (towhich the toner image is fixed) outside the image forming apparatus 1.

FIG. 2 is a block diagram showing a control system of the image formingapparatus 1 according to the first embodiment. The printing control unit100 (i.e., a controller) is connected to the LED head 3, a toner imageformation power source 7, a feeding motor power source 17, a fixingmotor power source 20, the writing sensor 8, an ejection sensor 9, afixing roller thermistor 62 (i.e., a first temperature detection unit),a pressure roller thermistor 65 (i.e., a second temperature detectionunit), and a heater power source 16.

The toner image formation power source 7 is connected to the toner imageforming unit 5 to supply electric power to the toner image forming unit5. The feeding motor power source 17 is connected to a medium feedingmotor 18 to supply electric power to the medium feeding motor 18. Thefixing motor power source 20 is connected to a fixing unit motor 21 tosupply electric power to the fixing unit motor 21. The heater powersource 16 is connected to a fixing heater 61 of the fixing unit 6.

The printing control unit 100 controls respective components of theimage forming apparatus 1 so as to perform an image forming operation.The LED head 3 emits light according to image data to expose the surfaceof the photosensitive drum 51 of the toner image forming unit 5. Thetoner image formation power source 7 applies voltages to the toner imageforming unit 5. For example, the toner image formation power source 7includes a charging power source that applies a charging voltage to thecharging roller 52, a developing power source that applies a developingvoltage to the developing unit 53, and a transfer power source thatapplies a transfer voltage to the transfer member 54. The fixing unitmotor 21 is driven by the electric power supplied by fixing motor powersource 20, and causes a fixing roller 64 (described later) of the fixingunit 6 to rotate.

The writing sensor 8 is configured to detect a position of the medium Malong the medium feeding path 2. The fixing unit 6 includes a fixingroller 64 (i.e., a fixing member), a pressure roller 63 (i.e., apressure member) pressed against the fixing roller 64 to form a nipportion, and a fixing heater 61 (i.e., a heating member) for heating thefixing roller 64. The heater power source 16 supplies electric power tothe fixing heater 61. The fixing roller thermistor 62 (i.e., a firsttemperature detection unit) detects a temperature of the fixing roller64 of the fixing unit 6. The pressure roller thermistor 65 (i.e., asecond temperature detection unit) detects a temperature of the pressureroller 63 of the fixing unit 6.

The printing control unit 100 includes a motor control unit 101, a speedsetting unit 102, a temperature detection unit 103, a temperaturedifference calculation unit 106, a heating control unit 104 and acomparison unit 105. The motor control unit 101 controls electric powersupply to the feeding motor power source 17 and the fixing motor powersource 20 so as to control operations of the medium feeding motor 18 andthe fixing unit motor 21. The motor control unit 101 controls electricpower supply to the feeding motor power source 17 and the fixing motorpower source 20 based on a rotation speed V which is set by the speedsetting unit 102.

The speed setting unit 102 (i.e., a rotation speed control unit)controls the rotation speed V of the fixing unit motor 21 according tooperating conditions of the image forming apparatus 1. To be morespecific, the speed setting unit 102 sets the rotation speed V of thefixing unit motor 21 before the medium M starts passing through thefixing unit 6 to a rotation speed Vprn based on a temperature differenceΔT0 between temperatures of the fixing roller 64 and the pressure roller63 (steps S104 and S111 in FIG. 5). Further, the speed setting unit 102calculates an optimum pre-arrival rotation speed V_(A) (step S107 inFIG. 5). The speed setting unit 102 sets the rotation speed V of thefixing unit motor 21 to the calculated optimum pre-arrival rotationspeed V_(A) (step S109 in FIG. 5).

The temperature detection unit 103 detects surface temperatures of thefixing roller 64 (i.e., an upper roller) and the pressure roller 63(i.e., a lower roller) using the fixing roller thermistor 62 and thepressure roller thermistor 65. The temperature difference calculatingunit 106 calculates the temperature difference ΔT0 between surfacetemperatures of the fixing roller 64 and the pressure roller 63. Theheating control unit 104 controls the heater power source 16 so as tokeep a temperature of the fixing unit 6 within a fixing-enablingtemperature range (i.e., a printing-enabling temperature range). To bemore specific, the heating control unit 104 determines whether thetemperature detected by the fixing roller thermistor 62 is within thepredetermined fixing-enabling temperature range. Based on adetermination result, the heating control unit 104 increases thetemperature of the fixing roller 64 by supplying electric power to thefixing heater 61 from the heater power source 16, or decreases thetemperature of the fixing roller 64 to decrease by stopping supplying ofthe electric power to the fixing heater 61 from the heater power source16. The comparison unit 105 compares information (for examples, thetemperatures of the fixing roller 64 and the pressure roller 63)according to instruction from the printing control unit 100.

FIG. 3 is a perspective view showing a configuration of the fixing unit6 according to the first embodiment. The fixing unit 6 includes thefixing roller 64 as a fixing member, the fixing heater 61 as a heatingunit, the pressure member 63 as a pressure member, the fixing rollerthermistor 62 as a first temperature detection unit, and the pressureroller thermistor 65 as a second temperature detection unit. In anexample shown in FIG. 3, the fixing roller 64 is disposed above thepressure roller 63. The fixing roller 64 is configured to supply heat tothe medium M and convey the medium M. The fixing heater 61 is configuredto heat the fixing roller 64. The fixing roller thermistor 62 isconfigured to detect the surface temperature of the fixing roller 64.The pressure roller thermistor 65 is configured to detect the surfacetemperature of the pressure roller 63.

The fixing roller 64 has a cylindrical shape, and includes a hollowcylindrical metal core in which the fixing heater 61 is provided. Thepressure roller 63 (for applying pressure to the medium M) is pressedagainst the fixing roller 64 to form a nip portion between the fixingroller 64 and the pressure roller 63. The fixing roller 64 and thepressure roller 63 rotate as shown by arrows A and A′ so that the mediumM passes through the nip portion. The fixing heater 61 is connected tothe heater power source 16. The heater power source 16 is connected tothe printing control unit 100 as described above. The temperaturecalculating unit 106 of the printing control unit 100 calculates thetemperature difference ΔT0 (i.e., an upper/lower temperature differenceΔT0) between the fixing roller 64 and the pressure roller 63 based onthe temperatures detected by the fixing roller thermistor 62 and thepressure roller thermistor 65.

FIG. 4A is a longitudinal sectional view showing the fixing unit 6according to the first embodiment. FIG. 4B is a cross sectional viewtaken along a line 4B-4B in FIG. 4A at a center of the fixing unit 6 ina longitudinal direction. FIG. 4C is a cross sectional view taken alonga line 4C-4C in FIG. 4A at an end portion of the fixing unit 6 in thelongitudinal direction. The fixing unit 6 includes the fixing roller 64,the pressure roller 63, ball bearings 66 (i.e., rotation supportingmembers) and a gear 67 (i.e., a driving force transmission unit). Theball bearings 66 rotatably support the fixing roller 64 and the pressureroller 63. The gear 67 is provided for transmitting a driving force fromthe fixing unit motor 21 to the fixing roller 64.

The fixing roller 64 contacts the pressure roller 63 to form the nipportion therebetween. The fixing heater 61 is mounted inside the fixingroller 64 in a non-contact manner. The fixing roller thermistor 62 isprovided so as to contact the surface of the fixing roller 64. Thepressure roller thermistor 65 is provided so as to contact the surfaceof the pressure roller 63. In this regard, it is also possible toprovide the fixing heater 61 so as to contact the fixing roller 64.Further, it is also possible to provide the fixing roller thermistor 62so as not to contact the surface of the fixing roller 64. It is alsopossible to provide the pressure roller thermistor 65 so as not tocontact the surface of the pressure roller 63.

The ball bearings 66 are provided on both ends of the fixing roller 64and both ends of the pressure roller 63. The gear 67 is provided on anend of the fixing roller 64. For example, the fixing roller 64 includesa metal core (i.e., a base body) having a diameter of 30 mm formed of aniron tube, and an elastic layer having a thickness of 1 mm formed ofsilicone rubber. The metal core of the fixing roller 64 is rotatablysupported by the ball bearings 66 at both ends. The gear 67 as thedriving force transmission unit is fixed to one end of the metal core ofthe fixing roller 64.

The fixing unit motor 21 is constituted by, for example, a pulse motor.The fixing unit motor 21 of this embodiment has a control-pulsegenerator. When the printing control unit 100 provides the fixing unitmotor 21 with electric power and clock signal having a frequency (i.e.,a clock frequency), the fixing unit motor 21 rotates at the rotationspeed V corresponding to the clock frequency. The printing control unit100 controls the rotation speed V of the fixing unit motor 21 bycontrolling the clock frequency. The pressure roller 63 is pressedagainst the fixing roller 64 by a resilient member such as a spring orthe like. The nip portion is formed between the pressure roller 63 andthe fixing roller 64. Therefore, when the fixing roller 64 rotates, thepressure roller 63 also rotates following the rotation of the fixingroller 64.

Each of the fixing roller thermistor 62 and the pressure rollerthermistor 65 is formed of an element whose resistance varies dependingon a temperature. The temperature detection unit 103 of the printingcontrol unit 100 obtains the temperatures detected by the fixing rollerthermistor 62 and the pressure roller thermistor 65 based on theresistances of the fixing roller thermistor 62 and the pressure rollerthermistor 65. The fixing roller thermistor 62 contacts the surface ofthe fixing roller 64, and the pressure roller thermistor 65 contacts thesurface of the pressure roller 63. The temperature detection unit 103detects the temperatures of the fixing roller 64 and the pressure roller63 by detecting outputs the thermistors 62 and 65. In this embodiment,each of the fixing roller thermistor 62 and the pressure rollerthermistor 65 is formed of an element whose resistance decreases as atemperature increases.

The fixing heater 61 is a heating element that generates heat whensupplied with electric power from a utility power source or the like.For example, the fixing heater 61 is formed of a halogen heater. Avoltage applied to the fixing heater 61 is, for example, 100 V. Anoutput of the fixing heater 61 is, for example, 800 W. A component ofthe fixing roller 64 has a relatively large heat capacity. It takes timefor heat to be transferred from an inner surface to an outer surface ofthe fixing roller 64. Therefore, there is a delay after the fixingheater 61 starts generating heat (i.e., after the fixing heater 61starts heating the metal core of the fixing roller 64) and before thesurface temperature of the fixing roller 64 starts increasing.

An operation of the fixing unit 6 according to the first embodiment willbe described with reference to FIGS. 2 and 5. When the printing controlunit 100 receives no print command (i.e., when the image formingapparatus 1 is in a standby state), the heating control unit 104 of theprinting control unit 100 keeps the fixing unit 6 at a temperature (forexample, 195° C.) at which fixing can be well performed so that imageformation can be started as soon as receiving print command. In thisstate, the fixing roller 64 does not rotate.

When the printing control unit 100 receives the print command, theprinting control unit 100 decides whether the temperature of the fixingroller 64 is in a fixing-enabling temperature range (described later).When the printing control unit 100 decides that the temperature of thefixing roller 64 is not within the fixing-enabling temperature range,the printing control unit 100 does not start feeding the medium M untilthe temperature of the fixing roller 64 reaches the fixing-enablingtemperature range. When the printing control unit 100 decides that thetemperature of the fixing roller 64 is within the fixing-enablingtemperature range, the printing control unit 100 causes the mediumfeeding unit 4 to start feeding the medium M by supplying electric powerto the medium feeding motor 18 from the feeding motor power source 17 insynchronization with image formation. Therefore, the medium M is fedalong the medium feeding path 2 toward the toner image forming unit 5.

The printing control unit 100 causes the LED head 3 to emit lightaccording to image data to expose the surface of the photosensitive drum51, and a latent image is formed on the surface of the photosensitivedrum 51. The latent image is developed by the developing unit 53, and atoner image is formed on the surface of the photosensitive drum 51. Thetoner image is transferred from the photosensitive drum 51 to the mediumM by the transfer member 54. The medium M is then fed to the fixing unit6, and the toner image is fixed to the medium M by application of heatand pressure. Thereafter, the medium M is ejected outside the imageforming apparatus 1.

A temperature control of the fixing unit 6 by the heat controlling unit104 will be herein described. The heating control unit 104 decideswhether the temperature detected by the fixing roller thermistor 62 isin the fixing-enabling temperature range (i.e., the printing-enablingtemperature range). When the printing control unit 100 decides that thetemperature of the fixing unit 6 is in the fixing-enabling temperaturerange, the motor control unit 101 of the printing control unit 100supplies electric power to the feeding motor power source 17 to therebydrive the medium feeding motor 18. That is, the medium feeding unit 41starts feeding the medium M.

The “fixing-enabling temperature range” is a temperature range in whicha toner image can be fixed to the medium M. The fixing-enablingtemperature range has a lower limit temperature T1 and an upper limittemperature T2. Further, a setting temperature Tprn is defined betweenthe lower limit temperature T1 and the upper limit temperature T2. Thelower limit temperature T1 is, for example, 175° C. The upper limittemperature T2 is, for example, 205° C. The setting temperature Tprn is,for example, 190° C. When the temperature of the fixing roller 64(detected by the fixing roller thermistor 62) is higher than the settingtemperature Tprn, the heating control unit 104 stops supplying electricpower to the fixing heater 61 from the heater power source 16 so thatthe temperature of the fixing roller 64 decreases. In other words, theheating control unit 104 performs a cool-down operation. When thetemperature of the fixing roller 64 (detected by the fixing rollerthermistor 62) is lower than the setting temperature Tprn, the heatingcontrol unit 104 supplies electric power to the fixing heater 61 fromthe heater power source 16 so that the temperature of the fixing roller64 increases. In other words, the heating control unit 104 performs awarm-up operation. That is, the heating control unit 104 keeps thetemperature of the fixing roller 64 in the fixing-enabling temperaturerange. Therefore, suitable amount of heat is applied to the medium M,and fixing failure is prevented.

A method of controlling the rotation speed V of the fixing unit motor 21according to the first embodiment will be described. FIG. 5 is aflowchart showing an operation for controlling the rotation speed of thefixing unit motor 21 according to the first embodiment.

First, the printing control unit 100 decides whether the printingcontrol unit 100 receives print command from a host device such as acomputer (S101). If the printing control unit 100 receives print command(YES in step S101), the printing control unit 100 proceeds to step S102.

In step S102, the temperature detection unit 103 of the printing controlunit 100 detects the temperatures of the fixing roller 64 and thepressure roller 63. The detected temperature of the fixing roller 64 isreferred to as a temperature Tup0. The detected temperature of thepressure roller 63 is referred to as a temperature Tlw0.

If the temperature Tup0 is within the fixing-enabling temperature range,the printing control unit 100 proceeds to step S103. If the temperatureTup0 is out of the fixing-enabling temperature range, the printingcontrol unit 100 waits until the temperature of the fixing roller 64reaches the fixing-enabling temperature range.

Then, in step S103, the temperature difference calculating unit 106 ofthe printing control unit 100 calculates the temperature difference ΔT0between current temperatures of the fixing roller 64 and the pressureroller 63 based on the temperatures Tup0 and Tlw0 detected by thetemperature detection unit 103 using the following equation:ΔT0=Tup0−Tlw0

Next, in step S104, the printing control unit 100 extracts a requestedprinting speed from the print command sent from the host device. Therequested printing speed is referred to as a printing speed Vprn. Thespeed setting unit 102 of the printing control unit 100 sets therotation speed V of the fixing unit motor 21 to the printing speed Vprn.

Then, in step S105, the printing control unit 100 selects aspeed-change-decision criterion temperature difference ΔTth for decidingwhether or not to change the rotation speed V of the fixing unit motor21.

The “speed-change-decision criterion temperature difference ΔTth” is atemperature difference between the fixing roller 64 and the pressureroller 63 based on which decision on whether or not to change therotation speed V of the fixing unit motor 21 is performed. Thespeed-change-decision criterion temperature difference ΔTth is setaccording to the printing speed. For example, when the printing speedVprn is 200 mm/s, the speed-change-decision criterion temperaturedifference ΔTth is 50° C. When the printing speed Vprn is 125 mm/s, thespeed-change-decision criterion temperature difference ΔTth is 100° C.When the printing speed Vprn is 50 mm/s, the speed-change-decisioncriterion temperature difference ΔTth is 150° C.

A method of determining the speed-change-decision criterion temperaturedifference ΔTth will be described with reference to FIGS. 6 and 7.

FIG. 6 is a schematic view for illustrating a relationship between theupper/lower temperature difference ΔT0 and a surface temperaturechanging amount D from start of rotation of the fixing roller 64according to the first embodiment. The “upper/lower temperaturedifference ΔT0” is a difference between the detected temperatures of thefixing roller 64 and the pressure roller 63. The “surface temperaturechanging amount D from start of rotation” is a changing amount in thesurface temperature of the fixing roller 64 during a predetermined timeperiod after the fixing roller 64 starts rotation from the standbystate. FIG. 6 shows the relationship between the upper/lower temperaturedifference ΔT0 and the surface temperature changing amount D from thestart of rotation of the fixing roller 64 for difference printingspeeds.

When the upper/lower temperature difference ΔT0 is large, thetemperature difference between the fixing roller 64 and the pressureroller 63 is large. For example, the temperature of the fixing roller 64is 195° C., the temperature of the pressure roller 63 is 95° C., and theupper/lower temperature difference ΔT0 is 100° C. When the fixing roller64 rotates in this state, a large amount of heat is transferred from thefixing roller 64 to the pressure roller 63 per unit time. Therefore, thetemperature of the fixing roller 64 decreases by a large amount.

In contrast, when the upper/lower temperature difference ΔT0 is small,the temperature difference between the fixing roller 64 and the pressureroller 63 is small. For example, the temperature of the fixing roller 64is 195° C., the temperature of the pressure roller 63 is 150° C., andthe upper/lower temperature difference ΔT0 is 45° C. Therefore, a smallamount of heat is transferred from the fixing roller 64 to the pressureroller 63 per unit time. Thus, the temperature of the fixing roller 64decreases by a small amount.

Accordingly, the upper/lower temperature difference ΔT0 is proportionalto a negative value (−D) of the surface temperature changing amount Dfrom the start of rotation.

Further, when the rotation speed V of the fixing roller 64 changes, thesurface temperature changing amount D from the start of rotation of thefixing roller 64 also changes. In other words, when the rotation speed Vof the fixing roller 64 becomes high, a frequency with which the fixingroller 64 and the pressure roller 63 contact each other increases. As aresult, a larger amount of heat is transferred from the fixing roller 64to the pressure roller 63. Therefore, for the same upper/lowertemperature difference ΔT0, the amount of heat transferred from thefixing roller 64 to the pressure roller 63 (i.e., the amount of heatdrawn from the fixing roller 64) per unit time increases as the rotationspeed V increases. In other words, the decrease in temperature of thefixing roller 64 becomes large as the rotation speed V increases. Incontrast, when the rotation speed V becomes low, the amount of heattransferred from the fixing roller 64 to the pressure roller 63 per unittime becomes small.

FIG. 7 is a schematic view for illustrating a relationship among theupper/lower temperature difference ΔT0, the surface temperature changingamount D from the start of rotation, a heat input amount P, a heatstorage amount Q at start of medium passing, and thespeed-change-decision criterion temperature difference ΔTth according tothe first embodiment.

The “heat input amount P (W)” is an amount of heat input into the fixingroller 64 by the heat control unit 104 after the fixing roller 64 startsrotating and before the medium M reaches the fixing unit 6. In otherwords, the heat input amount P (W) is an amount of heat to increase thetemperature of the fixing roller 64 to the temperature at which fixingcan be performed. The heat input amount P (W) is determined based on thesurface temperature changing amount D from the start of rotation of thefixing roller 64. The “heat storage amount Q (J) at start of mediumpassing” is an amount of heat having been stored in the fixing roller 64at a timing when the medium M starts passing through the fixing unit 6.The speed-change-decision criterion temperature difference ΔTth will bedescribed below. It is herein assumed that the printing speed V is low.However, the same can be said of a case where the printing speed V ishigh.

When the upper/lower temperature difference ΔT0 is large, the surfacetemperature changing amount D from the start of rotation of the fixingroller 64 becomes large in a negative (minus) direction as was describedwith respect to FIG. 6. In order to keep the surface temperature of thefixing roller 64 within the fixing-enabling temperature range (i.e., theprinting-enabling temperature range), the heating control unit 104increases the heat input amount P by supplying electric power to thefixing heater 61. Therefore, if the surface temperature changing amountD from the start of rotation of the fixing roller 64 is large, theheating control unit 104 is required to cause the fixing heater 61 togenerate more heat. Due to heat capacity and heat resistance of thefixing roller 64, the heat storage amount Q at the start of mediumpassing (i.e., the amount of heat having been stored in the fixingroller 64 when the medium M starts passing through the fixing unit 6)increases as the heat input amount P increases. The heat input amount Pand the heat storage amount Q at the start of medium passing areproportional to each other as shown in FIG. 7.

In contrast, when the upper/lower temperature difference ΔT0 is small,the surface temperature changing amount D from the start of rotation ofthe fixing roller 64 becomes small as shown in FIG. 6. The heatingcontrol unit 104 decreases the heat input amount P, and therefore theheat storage amount Q at the start of medium passing decreases.

As a result, it is understood that the heat storage amount Q at thestart of medium passing changes depends on a change in the upper/lowertemperature difference ΔT0 when the fixing roller 64 starts rotation. Inother words, if the upper/lower temperature difference ΔT0 is large, theheat storage amount Q at the start of medium passing becomes large. Ifthe upper/lower temperature difference ΔT0 is small, the heat storageamount Q at the start of medium passing becomes small.

When the medium M passes through the fixing unit 6, the fixing roller 64in a high temperature (for example, 180° C.) contacts the medium M in alow temperature (for example, 25° C.). Since the temperature differencebetween the fixing roller 64 and the medium M is large, a large amountof heat is transferred from the fixing roller 64 to the medium M. Inother words, a large amount of heat is drawn from the fixing roller 64,and the surface temperature of the fixing roller 64 is going to largelydecrease.

In such a case, if the heat storage amount Q at the start of mediumpassing is small, heat supplied to the surface of the fixing roller 64from inside decreases, and therefore the surface temperature of thefixing roller 64 largely decreases. Even if the heating control unit 104detects a decrease in the surface temperature of the fixing roller 64and causes the fixing heater 61 to generate more heat, it takes time forthe heat (generated by the fixing heater 61) to reach the surface of thefixing roller 64. Therefore, the surface temperature of the fixingroller 64 keeps decreasing until the heat reaches the surface of thefixing roller 64. As a result, the surface temperature of the fixingroller 64 largely decreases.

In contrast, if the heat storage amount Q at the start of medium passingis large, heat is transferred from the fixing roller 64 to the medium M,but heat is also supplied to the surface of the fixing roller 64 frominside. Therefore, decrease in the surface temperature of the fixingroller 64 is relatively small.

If the decrease in the surface temperature of the fixing roller 64 islarge, a sufficient amount of heat is not supplied to the medium M,which results in fixing failure. Therefore, in order to prevent fixingfailure, the heat storage amount Q at the start of medium passing needsto be large. A heat storage amount Q (at start of medium passing) neededto prevent fixing failure is referred to an optimum heat storage amountQ_(A). The heat input amount P corresponding to the optimum heat storageamount Q_(A) is referred to as a heat input amount P_(A). The surfacetemperature changing amount D (from the start of rotation) of the fixingroller 64 corresponds to the heat input amount P_(A) is referred to as asurface temperature changing amount D_(A) from the start of rotation.

The surface temperature changing amount D_(A) from the start of rotationchanges depending on the rotation speed V (also referred to as aprinting speed) of the fixing roller 64 in a printing process as shownin FIG. 7. Herein, description will be made of cases where the rotationspeed V is high and the rotation speed V is low.

When the rotation speed V is high in FIG. 7, if the upper/lowertemperature difference ΔT0 is larger than the speed-change-decisioncriterion temperature difference ΔTth, the heat storage amount Q largerthan the optimum heat storage amount Q_(A) can be obtained at therotation speed V. Therefore, it is not necessary to increase the heatstorage amount Q by increasing the rotation speed V.

In contrast, if the upper/lower temperature difference ΔT0 is smallerthan the speed-change-decision criterion temperature difference ΔTth inFIG. 7, it is necessary to increase the heat storage amount Q byincreasing the rotation speed V. The upper/lower temperature differenceΔT0 based on which whether or not to change the rotation speed V isdecided is referred to as the speed-change-decision criteriontemperature difference ΔTth. The speed-change-decision criteriontemperature difference ΔTth when the rotation speed V is high (V_(H)) isexpressed as ΔTth [V_(H)].

Similarly, the speed-change-decision criterion temperature differenceΔTth when the rotation speed V is low (V_(L)) is expressed as ΔTth[V_(L)]. The speed-change-decision criterion temperature difference ΔTth[V_(L)] is larger than the speed-change-decision criterion temperaturedifference ΔTth [V_(H)]. In other words, the following equation issatisfied: ΔTth [V_(H)]<ΔTth [V_(L)]. The value of thespeed-change-decision criterion temperature difference ΔTth changesdepending on the rotation speed V.

This indicates that, when the rotation speed V is low, it is necessaryto increase the rotation speed V more than when the rotation speed V ishigh for the same upper/lower temperature difference ΔT0. In otherwords, when the rotation speed V is low, an amount of heat transferredfrom the fixing roller 64 to the pressure roller 63 is small, andtherefore it is necessary to rotate the fixing roller 64 at a higherspeed in order to increase the heat storage amount Q for the sameupper/lower temperature difference ΔT0.

As described above, the speed-change-decision criterion temperaturedifference ΔTth [Vprn] is determined according to the rotation speed ofthe fixing roller 64 (i.e., the printing speed).

Referring back to FIG. 5, in step S106, the printing control unit 100instructs the comparison unit 105 to compare the upper/lower temperaturedifference ΔT0 calculated by the temperature difference calculation unit106 and the speed-change-decision criterion temperature difference ΔTthselected by the printing control unit 100.

When the comparison unit 105 determines that the upper/lower temperaturedifference ΔT0 is smaller than or equal to the speed-change-decisioncriterion temperature difference ΔTth (i.e., ΔT0≦ΔTth), the printingcontrol unit 100 changes the pre-arrival rotation speed V (step S107).The pre-arrival rotation speed V is the rotation speed V of the fixingunit motor 21 before the medium M reaches the fixing unit 6.

When the comparison unit 105 determines that the upper/lower temperaturedifference ΔT0 is larger than the speed-change-decision criteriontemperature difference ΔTth (i.e., ΔT0>ΔTth), the printing control unit100 does not change the rotation speed V (step S112).

In step S107, the speed setting unit 102 calculates the optimumpre-arrival rotation speed V_(A) using the following equation:V _(A) =A×ΔT0+B

In this equation, A and B are coefficients needed for calculating theoptimum pre-arrival rotation speed V_(A) based on the upper/lowertemperature difference ΔT0. The coefficients A and B are determined byexperiments. For example, the coefficient A is −1.5, and the coefficientB is 275.

When the requested printing speed Vprn of the fixing roller 64 are both50 mm/s (corresponding to the rotation speed V_(L)), and when theupper/lower temperature difference ΔT0 is 100° C., thespeed-change-decision criterion temperature difference ΔTth [50 mm/s] is150° C. In this case, the upper/lower temperature difference ΔT0 issmaller than speed-change-decision criterion temperature difference ΔTth(i.e., ΔT0<ΔTth) in step S106, and therefore it is decided that thepre-arrival rotation speed V needs to be changed. From the abovedescribed equation, the optimum pre-arrival rotation speed V_(A) isdetermined to be −1.5×100+275=125 mm/s in step S107.

A calculating method of the optimum pre-arrival rotation speed V_(A)will be described with reference to FIG. 8. FIG. 8 is a schematic viewshowing the method of calculating the optimum pre-arrival rotation speedV_(A) according to the first embodiment. FIG. 8 shows a relationshipbetween the upper/lower temperature difference ΔT0 and the optimumpre-arrival rotation speed V_(A) providing the optimum heat storageamount Q_(A). This is obtained by determining the upper/lowertemperature differences ΔT0 providing the optimum surface temperaturechanging amount D_(A) (see FIG. 7) from the start of rotation fordifferent rotation speeds V. From FIG. 8, it is understood that, as theupper/lower temperature difference ΔT0 becomes smaller, the optimumpre-arrival rotation speed V_(A) becomes higher (faster).

The optimum pre-arrival rotation speed V_(A) is determined as describedbelow. It is herein assumed that the upper/lower temperature differenceΔT0 is “dT_(A)” (FIG. 7) and the printing speed is V_(L) (i.e., a lowspeed). In this case, the upper/lower temperature difference ΔT0 issmaller than the speed-change-decision criterion temperature differenceΔTth [V_(L)], and therefore it is necessary to increase the heat storageamount by increasing the rotation speed of the fixing roller 64. Therotation speed V required in this case is a middle rotation speed V_(M)corresponding to dT_(A) in FIG. 8. It is understood from FIG. 8 that theoptimum heat storage amount Q_(A) is obtained by rotating the fixingroller 64 at the middle rotation speed V_(M) higher than the low speedV_(L).

Referring back to FIG. 5, in step S108, an amount of time required forthe medium M to reach the fixing unit 6 is calculated based on aposition and a feeding speed of the medium M. The calculated time isexpressed as T_(arrive). The printing control unit 100 instructs thecomparison unit 105 to compare the calculated time T_(arrive) and apredetermined time T_(const). If the calculated time T_(arrive) issmaller than or equal to and the predetermined time T_(const) (i.e.,T_(arrive)≦T_(const)), the printing control unit 100 proceeds to stepS109.

In this regard, the predetermined time T_(const) is an amount of time inwhich the temperature of the fixing roller 64 decreases (due to thechange in the rotation speed V) and returns to the same temperature asthat immediately before the change in the rotation speed V occurs. Thepredetermined time T_(const) does not depend on the rotation speed V,but is determined based on heat characteristic of the component of thefixing unit 6. For example, the predetermined time T_(const) is 3.0seconds. In this step S108, a timing of changing the rotation speed V ischanged according to the printing speed in order to keep the optimumheat storage amount Q_(A) (at the start of medium passing) even if therotation speed V is low.

The reason will be described below. An amount of time after the mediumfeeding unit 41 starts feeding the medium M and before the medium Mstarts passing through the fixing unit 6 is different depending onwhether the printing speed Vprn is high or low. When the printing speedVprn is high, the amount of time after the medium feeding unit 41 startsfeeding the medium M and before the medium M starts passing through thefixing unit 6 is shorter. Therefore, when the printing speed Vprn islow, if the timing of changing the rotation speed V is performed at thesame time when the printing speed Vprn is high, the temperature of thefixing roller 64 may return from the decreased temperature until themedium M starts passing through the fixing unit 6. That is, thetemperature of the fixing roller 64 may reach closer to the settingtemperature closer than when the printing speed Vrpn is high. As aresult, heat input amount P decreases, and the heat storage amount Q atthe start of medium passing may decrease.

Therefore, the rotation speed V is changed at different timingsdepending on the printing speed Vprn. More specifically, the rotationspeed of the fixing roller 64 is changed from the printing speed Vprn tothe optimum pre-arrival rotation speed V_(A) at a timing T_(const)before the medium M reaches the fixing unit 6.

In step S109, the speed setting unit 102 sets the rotation speed V ofthe fixing unit motor 21 to the optimum pre-arrival rotation speed V_(A)based on the calculation result of the optimum pre-arrival rotationspeed V_(A).

In step S110, the printing control unit 100 decides whether the medium Mreaches the fixing unit 6 or not based on output of the writing sensor8. This is performed as described below.

When the printing control unit 100 detects that a leading edge of themedium M reaches a position of the writing sensor 8 based on change inoutput of the writing sensor 8, the printing control unit 100 startscounting time. Since a distance (i.e., a medium feeding distance) fromthe writing sensor 8 to the fixing unit 6 is given, an amount of timerequired for the medium M to proceed from the position of the writingsensor 8 to the fixing unit 6 is calculated by dividing the givendistance by the medium feeding speed. Therefore, by counting the timeafter the leading edge of the medium M reaches the writing sensor 8, itis possible to detect that the medium M reaches the fixing unit 6.

In step S111, when the printing control unit 100 detects that the mediumM reaches the fixing unit 6, the speed setting unit 102 sets therotation speed V of the fixing unit motor 21 to the printing speed Vprn(i.e., V=Vprn).

Here, although it is described that the speed setting unit 102 sets therotation speed V of the fixing unit motor 21 to the printing speed Vprnwhen the medium M reaches the fixing unit 6, this embodiment is notlimited to such an arrangement. For example, in step S110, it is alsopossible that the printing control unit 100 decides whether apredetermined timing before the medium M reaches the fixing unit 6 hascome. Then, the printing control unit 100 changes the rotation speed Vto the printing speed Vprn. This is advantageous because the amount oftime required for the medium to reach the fixing unit 6 (determined bythe above described calculation) may include slight error.

In step S112, the printing control unit 100 performs the fixing process.

Using the above described processes, the necessary heat storage amount Qof the fixing roller 64 can be obtained for different printing speedseven when the upper/lower temperature difference ΔT0 is small.Therefore, the temperature of the fixing roller 64 can be prevented fromexcessively decreasing. As a result, fixing failure can be prevented.

Here, an operation of comparison example will be described. In thecomparison example, the rotation speed V of the fixing unit motor 21 isconstant (Vprn).

FIGS. 9A through 9F are timing charts showing an operation of the fixingunit 6 of the comparison example when the upper/lower temperaturedifference ΔT0 is large. FIGS. 9G through 9L are timing charts showingan operation of the fixing unit of the comparison example when theupper/lower temperature difference ΔT0 is small. The printing speed Vprnis set to the low speed V_(L) (=50 mm/s).

FIGS. 9A and 9G show the surface temperature of the fixing roller 64detected by the temperature detection unit 103. In FIGS. 9A and 9G, an“offset limit” indicates the lower limit temperature T1 (for example,175° C.) of the fixing-enabling temperature range. A “settingtemperature” indicates the setting temperature T_(prn) (for example,190° C.) of the fixing-enabling temperature range.

FIGS. 9B and 9H show the rotation speed V of the fixing unit motor 21controlled by the speed setting unit 102. FIGS. 9C and 9I show the heatinput amount P which is input into the fixing roller 64 under control ofthe heating control unit 104. FIGS. 9D and 9J show the heat storageamount Q of the fixing roller 64. FIGS. 9E and 9K show whether themedium M is passing through the fixing unit 6 or not. FIGS. 9F and 9Lshow whether the writing sensor 8 detects the medium M (ON) or not(OFF).

In FIGS. 9A through 9L, “ST00” and “ST10” show periods in which theprinting control unit 100 detects presence or absence of print command(i.e., the printing control unit 100 is in a standby state). Theseperiods “ST00” and “ST10” correspond to the step S101 in the flowchartof FIG. 5. In these periods “ST00” and “ST10”, the fixing unit motor 21stops, the writing sensor 8 does not detect the medium M, and the mediumM does not pass through the fixing unit 6.

Further, in FIGS. 9A through 9L, “S1” indicates a timing when theprinting control unit 100 starts rotating the fixing unit motor 21. “S2”indicates a timing when the medium M starts passing through the fixingunit 6. A period ST01 (ST11) starts at the timing S1, and ends at thetiming S2. A period ST02 (ST12) starts at the timing S2.

When the printing control unit 100 receives the print command, theprinting control unit 100 causes the fixing roller 64 to rotate at theprinting speed Vprn. When the fixing roller 64 starts rotation, thesurface temperature of the fixing roller 64 decreases as shown in FIGS.9A and 9G. The fixing roller thermistor 62 detects the decrease in thesurface temperature of the fixing roller 64. Then, the printing controlunit 100 increases the heat input amount P as shown in FIGS. 9C and 9I.Therefore, the heat storage amount Q increases as shown in FIGS. 9D and9K. With this, the printing control unit 100 keeps the temperature ofthe fixing unit 6 at the setting temperature, and performs a fixingprocess when the medium M reaches the fixing unit 6.

When the upper/lower temperature difference ΔT0 is large as shown inFIGS. 9A through 9F, the decrease in the surface temperature of thefixing roller 64 from the start of rotation (i.e., in the period ST01)is large due to the upper/lower temperature difference ΔT0 even if therotation speed V (FIG. 9B) is low. The surface temperature of the fixingroller 64 becomes lower than the offset limit as shown in FIG. 9A, andtherefore the heat input amount P becomes large as shown in FIG. 9C (inthe period ST01). Therefore, when the medium M starts passing throughthe fixing unit 6, a necessary heat storage amount has been obtained.Further, after the medium M has passed through the fixing unit 6, thesurface temperature of the fixing roller 64 shows a small decrease, andis kept within the fixing-enabling temperature. Therefore, fixingfailure does not occur in the period ST02.

In contrast, when the upper/lower temperature difference ΔT0 is small asshown in FIGS. 9G through 9L, the decrease in the surface temperature ofthe fixing roller 64 from the start of rotation (i.e., in the periodST10) is smaller than when the upper/lower temperature difference ΔT0 islarge. Therefore, the heat input amount P becomes small as shown in FIG.91 (in the period ST11). When the medium M starts passing through thefixing unit 6, the heat input amount P becomes larger, and the heatstorage amount Q increases. However, since the heat storage amount Q hasbeen small, the necessary heat storage amount is not obtained.Therefore, a necessary heat storage amount is not obtained as shown inFIG. 9J. Further, after the medium M has passed through the fixing unit6, the surface temperature of the fixing roller 64 largely decreases,and becomes lower than the offset limit (i.e., the lower limit) of thefixing-enabling temperature. As a result, fixing failure occurs in theperiod ST12.

Next, an example of an operation of the first embodiment will bedescribed.

FIGS. 10A through 10F are timing charts showing an operation of thefixing unit 6 according to the first embodiment when the upper/lowertemperature difference ΔT0 is small. FIGS. 10A through 10F areillustrated similarly to FIGS. 9A through 9F. In FIGS. 10A through 10F,“ST20” indicates a period in which the printing control unit 100 detectspresence or absence of print command (i.e., the printing control unit100 is in the standby state) as the periods ST00 and ST10 in FIGS. 9Aand 9G. “S1” indicates a timing when the printing control unit 100starts rotating the fixing unit motor 21. “S2” indicates a timing whenthe medium M starts passing through the fixing unit 6. A period ST21starts at the timing S1, and ends at the timing S2. A period ST22 startsat the timing S2.

When the printing control unit 100 receives the print command, theprinting control unit 100 causes the temperature detection unit 103 todetect the temperatures of the fixing roller 64 and the pressure roller63. The temperature detection unit 103 detects the temperatures of thefixing roller 64 and the pressure roller 63 by means of the fixingroller thermistor 62 and the pressure roller thermistor 65. As shown inFIG. 10A, the surface temperature of the fixing roller 64 is in thefixing-enabling temperature range in the period ST21. Therefore,according to the instruction from the printing control unit 100, thetemperature calculation unit 106 calculates the upper/lower temperaturedifference ΔT0 based on the temperatures detected by the temperaturedetection unit 103. Then, the printing control unit 100 selects thespeed-change-decision criterion temperature difference ΔTth [Vprn] fordeciding whether it is necessary to change the rotation speed V of thefixing unit motor 21.

When the printing control unit 100 decides that it is necessary tochange the rotation speed V, the speed setting unit 102 calculates theoptimum pre-arrival rotation speed V_(A) based on the selectedspeed-change-decision criterion temperature difference ΔTth and theprinting speed. The printing control unit 100 causes the speed settingunit 102 to rotate the fixing roller 64 at the rotation speed Vprn(i.e., a first rotation speed) (FIG. 10B). Further, the printing controlunit 100 causes the heating control unit 104 to control the heater powersource 16 so as to bring the surface temperature of the fixing roller 64within the fixing-enabling temperature.

When a remaining time before the medium M reaches the fixing unit 6becomes less than or equal to T_(const), the motor control unit 101causes the fixing unit motor 21 to rotate at the optimum pre-arrivalrotation speed V_(A) (i.e., a second rotation speed) set by the speedsetting unit 102. The rotation speed V_(A) (FIG. 10B) is sufficientlyhigh, and therefore the heat input amount P becomes large as shown inFIG. 10C, and the sufficient heat storage amount Q at the start ofmedium passing is obtained as shown in FIG. 10D (in a period ST21). Thiscan be understood from the relationship shown in FIG. 7.

Thereafter, when the printing control unit 100 detects that the medium Mreaches the fixing unit 6, the speed setting unit 102 causes the motorcontrol unit 101 to change the rotation speed V (FIG. 10B) of the fixingunit motor 21 to the printing speed Vprn. As a result, necessary andsufficient heat storage amount Q at the start of medium passing can beobtained. Therefore, even if the amount of heat transferred from thefixing roller 64 to the medium M becomes large immediately after themedium M starts passing through the fixing unit 6, the decrease in thesurface temperature of the fixing roller 64 can be suppressed.Therefore, fixing failure can be prevented in the period ST22.

As described above, according to the first embodiment of the presentinvention, the decrease in the temperature of the fixing roller 64immediately after the medium M starts passing through the fixing unit 6can be suppressed. Accordingly, the printing failure can be preventedeven when the upper/lower temperature difference ΔT0 is small.

Second Embodiment

FIG. 11 is a block diagram showing a control system of an image formingapparatus according to the second embodiment of the present invention.In the second embodiment, components that are the same as those of thefirst embodiment are assigned the same reference numerals. The imageforming apparatus of the second embodiment is different from that of thefirst embodiment in the printing control unit 200. More specifically,the printing control unit 200 employs a different speed setting methodfrom that of the printing control unit 100 of the first embodiment. Theprinting control 200 includes a speed setting unit 202 which isdifferent from the speed setting unit 102 of the first embodiment.Further, unlike the image forming apparatus of the first embodiment, theimage forming apparatus of the second embodiment includes anenvironmental temperature sensor 210 as an environmental temperaturedetection unit (i.e., a third temperature detection unit).

The environmental temperature sensor 210 is mounted in the image formingapparatus 1, and is connected to a temperature detection unit 203 of theprinting control unit 200. The environmental temperature sensor 210detects the temperature in the image forming apparatus 1. Thetemperature detection unit 203 receives information on the surfacetemperatures of the fixing roller 64 and the pressure roller 63 from thefixing roller thermistor 62 and the pressure roller thermistor 65, andalso receives information on the temperature in the image formingapparatus 1 from the environmental temperature sensor 210. Othercomponents of the second embodiment are the same as those of the firstembodiment.

FIG. 12 is a flowchart showing an operation for controlling the rotationspeed of the fixing unit motor 21 according to the second embodiment.The operation of the second embodiment will be described with referenceto FIG. 12. Steps S201, S202, S203 and S204 are the same as the stepsS101, S102, S103 and S104, and explanations thereof are omitted.

In step S205, the temperature detection unit 203 of the printing controlunit 200 obtains an environmental temperature Tenv (i.e., a detectionresult) from the environmental temperature sensor 210.

In step S206, the printing control unit 200 selects thespeed-change-decision criterion temperature difference ΔTthcorresponding to the environmental temperature Tenv in order to decidewhether it is necessary to change the rotation speed V of the fixingunit motor 21.

When the environmental temperature Tenv is high (for example, higherthan or equal to 30° C.), the printing control unit 200 selects aspeed-change-decision criterion temperature difference ΔTth1 [Vprn].

When the environmental temperature Tenv is normal (for example, higherthan or equal to 15° C. but lower than 30° C.), the printing controlunit 200 selects a speed-change-decision criterion temperaturedifference ΔTth2 [Vprn].

When the environmental temperature Tenv is low (for example, lower than15° C.), the printing control unit 200 selects a speed-change-decisioncriterion temperature difference ΔTth3 [Vprn].

The speed-change-decision criterion temperature differences ΔTth1, ΔTth2and ΔTth3 are determined by experiments.

For example, when the rotation speed V is 200 mm/s, thespeed-change-decision criterion temperature difference ΔTth1 [200 mm/s]is 20° C. When the rotation speed V is 125 mm/s, thespeed-change-decision criterion temperature difference ΔTth1 [125 mm/s]is 50° C. When the rotation speed V is 50 mm/s, thespeed-change-decision criterion temperature difference ΔTth1 [50 mm/s]is 80° C.

Further, when the rotation speed V is 200 mm/s, thespeed-change-decision criterion temperature difference ΔTth2 [200 mm/s]is 50° C. When the rotation speed V is 125 mm/s, thespeed-change-decision criterion temperature difference ΔTth2 [125 mm/s]is 100° C. When the rotation speed V is 50 mm/s, thespeed-change-decision criterion temperature difference ΔTth2 [50 mm/s]is 150° C.

Further, when the rotation speed V is 200 mm/s, thespeed-change-decision criterion temperature difference ΔTth3 [200 mm/s]is 90° C. When the rotation speed V is 125 mm/s, thespeed-change-decision criterion temperature difference ΔTth3 [125 mm/s]is 150° C. When the rotation speed V is 50 mm/s, thespeed-change-decision criterion temperature difference ΔTth3 [50 mm/s]is 210° C.

Here, the speed-change-decision criterion temperature difference ΔTthwill be described with reference to FIGS. 13 and 14.

FIG. 13 is a schematic view for illustrating a relationship between aheat storage amount Q at the start of medium passing and a surfacetemperature changing amount D from the start of rotation of the fixingroller 64 for different environmental temperatures. The environmentaltemperature is considered to be almost the same as a temperature of themedium M. When the temperature of the medium M is low, the temperaturedifference between the medium M and the fixing roller 64 is larger thanwhen the temperature of the medium M is high. Therefore, an amount ofheat transferred from the fixing roller 64 to the medium M per unit timebecomes larger, and the temperature of the fixing roller 64 tends todecrease largely. Accordingly, for the same heat storage amount Q, whenthe temperature of the medium M is low, the surface temperature of thefixing roller 64 largely decreases than when the temperature of themedium M is high.

FIG. 14 is a schematic view showing a relationship between theupper/lower temperature difference ΔT0, the surface temperature changingamount D from the start of rotation, the heat input amount P, the heatstorage amount Q at the start of medium passing, and thespeed-change-decision criterion temperature difference ΔTth. As comparedwith FIG. 7 described in the first embodiment, FIG. 14 shows that theheat storage amount Q of the fixing roller 64 varies depending on thetemperature of the medium M (i.e., the environmental temperature). Inthis regard, FIG. 7 of the first embodiment shows the optimum heatstorage amount Q_(A) when the temperature of the medium M is normal.FIG. 14 shows the optimum heat storage amounts Q_(A) when thetemperature of the medium M is low, normal and high.

In order that the temperature changes after the medium M has passedthrough the fixing unit 6 are the same, a larger heat storage amount Qis needed when the temperature of the medium M is low than when thetemperature of the medium M is high (i.e., Q_(A1)<Q_(A3)). As a result,the speed-change-decision criterion temperature difference ΔTth differsdepending on the temperatures of the medium M.

In FIG. 14, H1 represents ΔTth1 [V_(H)], H2 represents ΔTth2 [V_(H)],and H3 represents ΔTth3 [V_(H)]. Further, M2 represents ΔTth2 [V_(M)],and L2 represents ΔTth2 [V_(L)]. In the example of FIG. 14, thefollowing relationship is satisfied: H1<H2<H3<M2<L2. That is, thespeed-change-decision criterion temperature difference ΔTth differsdepending on the printing speed V_(L), V_(M) and V_(H). In this way, thespeed-change-decision criterion temperature difference ΔTth [Vprn]corresponding to the printing speed Vprn can be determined.

Referring back to FIG. 12, in step S207, the printing control unit 200causes the comparison unit 105 to compare the upper/lower temperaturedifference ΔT0 (calculated by the temperature difference calculationunit 106) and the speed-change-decision criterion temperature differenceΔTth selected by the printing control unit 200.

When the upper/lower temperature difference ΔT0 is smaller than or equalto the speed-change-decision criterion temperature difference ΔTth[Vprn, Tenv] (i.e., ΔT0≦ΔTth [Vprn, Tenv]), the printing control unit200 changes the rotation speed (steps S208 through S213).

When the upper/lower temperature difference ΔT0 is larger than thespeed-change-decision criterion temperature difference ΔTth [Vprn, Tenv](i.e., ΔT0>ΔTth [Vprn, Tenv]), the printing control unit 200 does notchange the rotation speed (step S214).

In step S208, the printing control unit 200 selects optimum speedcalculation coefficients A and B based on the environmental temperatureTenv. The speed setting unit 202 calculates the optimum pre-arrivalrotation speed V_(A) using an equation (step S209). The equation isselected based on the environmental temperature among the followingequations respectively determining the optimum pre-arrival rotationspeeds V_(A1), V_(A2) and V_(A3).

When the environmental temperature is high, the following equation isprovided: V_(A1)=A1×ΔT0+B1.

When the environmental temperature is normal, the following equation isprovided: V_(A2)=A2×ΔT0+B2.

When the environmental temperature is low, the following equation isprovided: V_(A3)=A3×ΔT0+B3.

The speed calculation coefficients A1, B1, A2, B2, A3 and B3 aredetermined by experiments. For example, A1 is −2.5, B1 is 250, A2 is−1.5, B2 is 200, A3 is −1.25, and B3 is 312.5.

Steps S209 through S214 are the same as the steps S107 through S112 ofthe first embodiment, and explanations thereof are omitted.

FIG. 15 is a schematic view showing a method of calculating the optimumpre-arrival rotation speed V_(A1), V_(A2) and V_(A3) for differentenvironmental temperatures according to the second embodiment. FIG. 15shows a relationship between the upper/lower temperature difference ΔT0and the optimum pre-arrival rotation speed V_(A1), V_(A2) and V_(A3)under the condition that the optimum heat storage amounts Q_(A1), Q_(A2)and Q_(A3) are obtained for respective environmental temperatures. Thisis obtained by determining upper/lower temperature differences ΔT0 thatprovides the surface temperature changing amounts D_(A1), D_(A2) andD_(A3) (from the start of rotation of the fixing roller 64) forrespective rotation speeds in FIG. 14. From FIG. 15, it is understoodthat the pre-arrival rotation speed V_(A) becomes higher (faster) as theenvironmental temperature in the image forming apparatus 1 detected bythe environmental temperature sensor 210 becomes lower. Other processesare the same as those of the first embodiment, and explanations thereofare omitted.

The operation of the second embodiment will be described with referenceto FIGS. 16A through 16L. FIGS. 16A through 16F are timing chartsshowing operations under a low temperature and low humidity environment(i.e., an LL environment) according to the second embodiment. FIGS. 16Gthrough 16L are timing charts showing operations under a hightemperature and high humidity environment (i.e., an HH environment)according to the second embodiment. In FIGS. 16A through 16F, theupper/lower temperature difference ΔT0 is small as described withreference to FIGS. 9A through 9F. FIGS. 16A through 16L are illustratedsimilarly to FIGS. 9A through 9L.

In FIGS. 16A through 16L, “ST50” and “ST60” indicate periods in whichthe printing control unit 200 detects presence or absence of printcommand (i.e., the image forming apparatus is in the standby state), andcorrespond to the step S201 in the flowchart of FIG. 12. Further, “S1”indicates a timing when the printing control unit 200 starts rotatingthe fixing unit motor 21. “S2” indicates a timing when the medium Mstarts passing through the fixing unit 6. A period ST51 (ST61) starts atthe timing S1, and ends at the timing S2. A period ST52 (ST62) starts atthe timing S2.

When the printing control unit 200 receives the print command, theprinting control unit 200 causes the temperature detection unit 203 todetect the temperatures of the fixing roller 64 and the pressure roller63. The temperature detection unit 203 detects the temperatures of thefixing roller 64 and the pressure roller 63 by means of the fixingroller thermistor 62 and the pressure roller thermistor 65. As shown inFIGS. 16A and 16G, the surface temperature of the fixing roller 64 is inthe fixing-enabling temperature range in the period ST51 (ST61).Therefore, according to the instruction from the printing control unit200, the temperature calculation unit 106 calculates the upper/lowertemperature difference ΔT0 based on the temperatures detected by thetemperature detection unit 203. Then, the printing control unit 200selects the speed-change-decision criterion temperature difference ΔTth[Vprn] for deciding whether it is necessary to change the rotation speedV of the fixing unit motor 21. When the printing control unit 200decides that it is necessary to change the rotation speed V, the speedsetting unit 202 calculates the optimum pre-arrival rotation speed V_(A)based on the selected speed-change-decision criterion temperaturedifference ΔTth and the printing speed.

Then, the printing control unit 200 causes the speed setting unit 202 torotate the fixing roller 64 at the rotation speed Vprn (FIGS. 16B and16H). Further, the printing control unit 200 causes the heating controlunit 204 to control the heater power source 16 so as to bring thesurface temperature of the fixing roller 64 within the fixing-enablingtemperature (FIGS. 16A and 16G). When a remaining time before the mediumM reaches the fixing unit 6 becomes less than or equal to T_(const), themotor control unit 101 causes the fixing unit motor 21 to rotate at theoptimum pre-arrival rotation speed V_(A) set by the speed setting unit202.

The rotation speed V_(A3) (FIG. 16B) is sufficiently high, and thereforethe surface temperature changing amount D from the start of rotation ofthe fixing roller 64 increases. Therefore, the heat input amount Pbecomes large (periods ST51 and ST61). In this regard, a larger heatstorage amount is needed under the low temperature and low humidityenvironment (FIG. 16D) than under the high temperature and high humidityenvironment (FIG. 16J). Therefore, the optimum pre-arrival rotationspeed V_(A3) under the low temperature and low humidity environment(FIG. 16B) is higher than the optimum pre-arrival rotation speed V_(A1)under the high temperature and high humidity environment (FIG. 16H). Inother words, the heat input amount P under the low temperature and lowhumidity environment (FIG. 16C) is larger than under the hightemperature and high humidity environment (FIG. 16I).

Thereafter, when the printing control unit 200 detects that the medium Mreaches the fixing unit 6 based on the detection result of the writingsensor 8, the speed setting unit 202 causes the motor control unit 101to change the rotation speed V (FIGS. 16B and 16H) of the fixing unitmotor 21 to the printing speed Vprn. Then, the medium M starts to be fedthrough the fixing unit 6.

In this regard, the heat storage amount Q under the low temperature andlow humidity environment (FIG. 16D) is larger than the heat storageamount Q under the high temperature and high humidity environment (FIG.16J) as described above. Therefore, even if the heat transferred fromthe fixing roller 64 to the medium M increases due to the lowtemperature of the medium M, the decrease in the temperature of thefixing roller 64 can be substantially the same as under the hightemperature and high humidity environment. As a result, the decrease inthe temperature of the fixing roller 64 immediately after the medium Mstarts passing through the fixing unit 6 can be reduced. That is, fixingfailure can be prevented.

Modification 1.

In the second embodiment, the target rotation speed V is changed basedon the optimum heat storage amount Q corresponding to the environmentaltemperature (which is considered to be substantially the same as thetemperature of the medium M). In this regard, it is also effective inpreventing fixing failure to change the optimum heat storage amount Qand the target rotation speed V based on a thickness of the medium M. Itis herein assumed that the environmental temperature is made constant.

FIG. 17 is a block diagram showing a control system of an image formingapparatus 1 according to Modification 1 of the second embodiment. Theimage forming apparatus 1 of Modification 1 includes a medium thicknesssetting unit 211 for setting the thickness of the medium M on whichprinting is to be performed. The medium thickness setting unit 211 isconnected to a printing control unit 300. The medium thickness settingunit 211 includes an input unit (i.e., an operation unit) operated by anoperator. The input unit includes buttons for designating one of a thinmedium (i.e., a thin sheet) and a thick medium (i.e., a thick sheet).The operator can set the thickness of the medium by pressing the thinmedium button or the thick medium button of the medium thickness settingunit 211. Alternatively, in the case where the print command sent fromthe host device (i.e., a host controller) includes information on thethickness of the medium M, the medium thickness setting unit 211 can bemounted in the printing control unit 300 and can be configured to detectthe thickness of the medium M based on the print command. Furthermore,the medium thickness setting unit 211 can be configured to automaticallydetect the thickness of the medium M using a thickness sensor (forexample, a pair of rollers between which the medium M is nipped).

A speed setting unit 302 mounted in the printing control unit 300 isdifferent from the speed setting unit 202 of the second embodiment. Thespeed setting unit 302 controls the rotation speed V based on theoptimum heat storage amount Q that changes according to the thickness ofthe medium M as shown in FIG. 18. Therefore, it becomes possible to keepconstant the decrease in the temperature of the fixing roller 64 afterthe medium M starts passing through the fixing unit 6 irrespective ofthe thickness of the medium M. Since the medium M has a constant surfacearea defined by international standard (for example, A4 size), a volumeof the medium M increases as the thickness of the medium M increases. Asthe volume of the medium M increases, a heat capacity of the medium Malso increases. As the heat capacity of the medium M increases, anamount of heat transferring from the fixing roller 64 to the medium Malso increase, and therefore decrease in the temperature of the fixingroller 64 becomes larger.

Therefore, in Modification 1, the rotation speed V is changed due to thethickness of the medium M. To be more specific, even if the upper/lowertemperature difference ΔT0 is the same, the rotation speed V is sethigher as the medium M becomes thicker. This increases the heat storageamount Q of the fixing roller 64 at start of medium passing (i.e., whenthe medium M start passing through the fixing unit 6), with the resultthat the decrease in the temperature of the fixing roller 64 is reduced.

FIG. 18 is a schematic view showing the upper/lower temperaturedifference ΔT0, the surface temperature changing amount D from the startof rotation, the heat input amount P, the heat storage amount Q at thestart of medium passing, and the speed-change-decision criteriontemperature difference ΔTth according to Modification 1. FIG. 18 showsthat necessary heat storage amount Q of the fixing roller 64 shown inFIG. 7 of the first embodiment varies depending on the thickness of themedium M. FIG. 7 of the first embodiment shows the optimum heat storageamount Q_(A) when the medium M is a thin sheet. In contrast, FIG. 18shows the heat storage amount Q_(A12) when the medium M is a thin sheetand the heat storage amount Q_(A32) when the medium M is a thick sheet.

In order to keep constant the surface temperature changing amount D fromthe start of rotation of the fixing roller 64, the necessary heatstorage amount Q_(A12) when the medium M is thick is larger than thenecessary heat storage amount Q_(A32) when the medium M is thin (i.e. n(i.e., Q_(A12)<Q_(A32)). Therefore, the speed-change-decision criteriontemperature difference ΔTth when the medium M is thick is different fromthe speed-change-decision criterion temperature difference ΔTth when themedium M is thin.

In FIG. 18, H12 represents ΔTth1 [V_(H)], H22 represents ΔTth2 [V_(H)],and H32 represents ΔTth3 [V_(H)]. Further, M22 represents ΔTth2 [V_(M)],and L22 represents ΔTth2 [V_(L)]. In the example shown in FIG. 18, thefollowing relationship is satisfied: H12<H22<H32<M22<L22. That is, thespeed-change-decision criterion temperature difference ΔTth differsdepending on the printing speed V_(H), V_(M) or V_(L).

In this way, the speed-change-decision criterion temperature differenceΔTth [Vprn] corresponding to the printing speed (rotation speed) can bedetermined.

Modification 2.

It is also effective in preventing fixing failure to change the targetrotation speed V and the optimum heat storage amount Q based on thenumber of the media M on which printing is to be performed. It is hereinassumed that the environmental temperature and the thickness of themedium M are respectively made constant.

FIG. 19 is a block diagram showing a control system of an image formingapparatus 1 according to Modification 2 of the second embodiment. Theimage forming apparatus 1 of Modification 2 includes a medium numberdetection unit 212 for detecting the number of the media M on whichprinting is to be performed. The medium number detection unit 212 ismounted in a printing control unit 400. When the print command sent fromthe host device (i.e., the host computer) includes information on thenumber of the media M on which printing is to be performed, the mediumnumber detection unit 212 detects the number of the media M based on theprint command. Alternatively, the medium number detection unit 212 canhave an input unit (i.e., an operation unit) operated by an operator,and the input unit can have a button (i.e., a number setting button) forsetting the number of the media M. In such a case, the medium numberdetection unit 212 can detect the number of the media M based on theuser's operation of the number setting button.

A speed setting unit 402 mounted in the printing control unit 400 isdifferent from the speed setting unit 202 of the second embodiment. Thespeed setting unit 402 controls the rotation speed V based on theoptimum heat storage amount Q that changes according to the number ofthe media M as shown in FIG. 20. Therefore, it becomes possible to keepconstant the decrease in the temperature of the fixing roller 64 afterthe medium M starts passing through the fixing unit 6 irrespective ofthe number of the media M. As the number of the media M on whichprinting is continuously performed increases, an amount of heat drawnfrom the fixing roller 64 increases. Therefore, an amount of heat(needed for fixing images) increases as the number of media M increases.Further, since it takes time for the heat (generated by the fixingheater 61) to reach the surface of the fixing roller 64 as describedabove, the temperature of the fixing roller 64 tends to furtherdecrease.

Therefore, in Modification 2, the rotation speed V is controlled basedon the number of media M on which printing is to be continuouslyperformed. To be more specific, even if the upper/lower temperaturedifference ΔT0 is the same, the rotation speed V is set higher as thenumber of media M increases. This increases the heat storage amount Q atthe start of medium passing, with the result that the decrease in thetemperature of the fixing roller 64 is reduced.

FIG. 20 is a schematic view showing the upper/lower temperaturedifference ΔT0, the surface temperature changing amount D from the startof rotation, the heat input amount P, the heat storage amount Q at thestart of medium passing, and the speed-change-decision criteriontemperature difference ΔTth according to Modification 2. FIG. 20 showsthat necessary heat storage amount Q of the fixing roller 64 shown inFIG. 7 of the first embodiment varies depending on the number of themedia M. FIG. 7 of the first embodiment shows the optimum heat storageamount Q_(A) when the number of media M is 1. In contrast, FIG. 20 showsthe heat storage amount Q_(A13) when the number of the media M is 1 andthe heat storage amount Q_(A33) when the number of the media M is 10 ormore.

In order to keep constant the surface temperature changing amount D fromthe start of rotation of the fixing roller 64, the necessary heatstorage amount Q_(A33) when the number of the media M is 10 or more islarger than the necessary heat storage amount Q_(A32) when the medium Mis 1 (i.e. Q_(A13)<Q_(A33)). Therefore, the speed-change-decisioncriterion temperature difference ΔTth differs depends on the number ofthe media M.

In FIG. 20, H13 represents ΔTth1 [VH], H23 represents ΔTth2 [VH], andH33 represents ΔTth3 [VH]. Further, M23 represents ΔTth2 [VM], and L23represents ΔTth2 [VL]. In the example shown in FIG. 20, the followingrelationship is satisfied: H13<H23<H33<M23<L23. Thespeed-change-decision criterion temperature difference ΔTth differsdepending on the printing speed V_(H), V_(M) or V_(L).

In this way, the speed-change-decision criterion temperature differenceΔTth [Vprn] corresponding to the printing speed (i.e., the rotationspeed) can be determined.

As described above, according to the second embodiment of the presentinvention, the heat storage amount at start of medium passing isincreased by increasing the rotation speed V taking into considerationthe decrease in the temperature of the fixing roller 64 after the mediumM reaches the fixing unit 6 (caused by the change in the environmentaltemperature, i.e., the temperature of the medium M). Therefore, thedecrease in the temperature of the fixing roller 64 immediately afterthe medium M starts passing through the fixing unit 6 can be reduced.Accordingly, fixing failure can be prevented even when the environmentaltemperature varies on condition that the upper/lower temperaturedifference ΔT0 is small.

In this regard, the above described Modifications 1 and 2 can also beapplied to the first embodiment.

In the above described embodiments, an electrophotographic printer hasbeen described as an example of the image forming apparatus. However,the present invention is also applicable to a facsimile machine, acopier, a multifunction peripheral or the like.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andimprovements may be made to the invention without departing from thespirit and scope of the invention as described in the following claims.

What is claimed is:
 1. An image forming apparatus comprising: a fixingmember configured to fix an image to a medium by heating the medium; aheating member configured to heat the fixing member; a pressure memberpressed against the fixing member so as to press the medium against thefixing member; a first temperature detection unit that detects atemperature of the fixing member; a second temperature detection unitthat detects a temperature of the pressure member; and a control unitthat controls a rotation speed of the fixing member; wherein the controlunit controls the rotation speed of the fixing member based on atemperature difference between the temperature detected by the firsttemperature detection unit and the temperature detected by the secondtemperature detection unit; wherein the control unit has a plurality ofselectable printing speeds that define rotation speeds of the fixingmember when the medium passes the fixing member, and has a plurality ofselectable speed-change-decision criterion temperature differencesrespectively corresponding to the printing speeds; and wherein thecontrol unit selects one of the speed-change-decision criteriontemperature differences according to the selected printing speed, anddetermines the rotation speed to be the same as or different from theprinting speed based on the selected speed-change-decision criteriontemperature difference.
 2. The image forming apparatus according toclaim 1, wherein the control unit controls the rotation speed of thefixing member when the temperature detected by the first temperaturedetection unit is within a fixing-enabling temperature range.
 3. Theimage forming apparatus according to claim 1, wherein the control unitincreases the rotation speed of the fixing member, as the temperaturedifference becomes smaller.
 4. The image forming apparatus according toclaim 3, wherein, when the temperature difference is smaller than orequal to a predetermined value, the control unit sets the rotation speedof the fixing member to a second rotation speed that is higher than afirst rotation speed at which a fixing process is performed.
 5. Theimage forming apparatus according to claim 4, wherein the control unitchanges the rotation speed of the fixing member from the first rotationspeed to the second rotation speed before the medium reaches the fixingmember.
 6. The image forming apparatus according to claim 5, wherein,when the temperature difference is smaller than or equal to apredetermined value, the control unit controls the rotation speed of thefixing member according to the temperature difference.
 7. The imageforming apparatus according to claim 6, wherein, when the temperaturedifference is expressed as ΔT0 and the rotation speed of the fixingmember is expressed as V_(A), the control unit sets the rotation speedV_(A) of the fixing member based on the following equation:V _(A) =A×ΔT0+B where A and B are constants.
 8. The image formingapparatus according to claim 1, further comprising a third temperaturedetection unit that detects a temperature in the image formingapparatus.
 9. The image forming apparatus according to claim 8, whereinthe control unit sets the rotation speed of the fixing member to ahigher speed, as the temperature detected by the third temperaturedetection unit becomes lower.
 10. The image forming apparatus accordingto claim 1, further comprising a medium thickness setting unit that setsa thickness of the medium.
 11. The image forming apparatus according toclaim 10, wherein the control unit sets the rotation speed of the fixingmember to a lower speed, as the thickness of the medium set at themedium thickness setting unit becomes thinner.
 12. The image formingapparatus according to claim 1, further comprising a medium numberdetection unit that detects a number of medium on which image formationis to be performed.
 13. The image forming apparatus according to claim12, wherein the control unit sets the rotation speed of the fixingmember to a higher speed, as the number of medium detected by the mediumnumber detection unit becomes larger.
 14. The image forming apparatusaccording to claim 1, wherein the rotation speed of the fixing memberbefore the medium reaches the fixing member differs depending on thetemperature difference.
 15. The image forming apparatus according toclaim 1, wherein the control unit changes a timing at which the controlunit changes the rotation speed from the printing speed.
 16. The imageforming apparatus according to claim 1, wherein the control unit selectsone of the speed-change-decision criterion temperature differencesaccording to the printing speeds and an environmental temperature. 17.The image forming apparatus according to claim 1, wherein the rotationspeed of the fixing member before the medium reaches the fixing memberdiffers depending on an environmental temperature.
 18. The image formingapparatus according to claim 1, wherein the speed-change-decisioncriterion temperature difference differs depending on a thickness of themedium.
 19. The image forming apparatus according to claim 1, whereinthe speed-change-decision criterion temperature difference differsdepending on a number of the medium on which printing is to becontinuously performed.
 20. An image forming apparatus comprising: afixing member heated by a heating member, the fixing member beingconfigured to fix an image to a medium by heating the medium; a pressuremember pressed against the fixing member so as to press the mediumagainst the fixing member; and a control unit that controls a rotationspeed of the fixing member; wherein the control unit controls a rotationspeed of the fixing member, the control unit causing the fixing memberto rotate at a higher speed, as a heat storage amount in the fixingmember becomes smaller; wherein the control unit has a plurality ofselectable printing speeds that define rotation speeds of the fixingmember when the medium passes the fixing member, and has a plurality ofselectable speed-change-decision criterion temperature differencesrespectively corresponding to the printing speeds; and wherein thecontrol unit selects one of the speed-change-decision criteriontemperature differences according to the selected printing speed, anddetermines the rotation speed to be the same as or different from theprinting speed based on the selected speed-change-decision criteriontemperature difference.
 21. The image forming apparatus according toclaim 20, wherein the control unit controls the rotation speed of thefixing member when the temperature of the fixing member is in afixing-enabling temperature range.